channeldb: add persistance of the onion blob

In this commit htlc channeldb representation have been augmented
with onion blob field, and (de)serialisaion functions have been changed
to make the onion blob persistant.

channeldb/channel_test.go

@@ -207,6 +207,7 @@ func TestOpenChannelPutGetDelete(t *testing.T) {
 			Amt:           10,
 			RHash:         key,
 			RefundTimeout: 1,
+			OnionBlob:       []byte("onionblob"),
 		},
 	}
 	if err := state.FullSync(); err != nil {